Pegylated interferon beta (immunomodulator) Pregnancy: eMC §4.6: a large amount of data (more than 1,000 pregnancy outcomes) from registries and post-marketing experience indicates no increased risk of major congenital anomalies after pre-conception exposure to interferon beta or exposure during the first trimester; experience in the second and third trimesters is very limited. Based on animal data there is a possibly increased risk of spontaneous abortion, although the currently available human data do not suggest an increased risk. If clinically needed, use may be considered during pregnancy. Breast-feeding - levels excreted in human milk are expected to be negligible and no harmful effects on the breastfed newborn/infant are anticipated; peginterferon beta-1a can be used during breast-feeding. Fertility - no human data; anovulatory effects were observed in animals at very high doses.

Peginterferon beta-1a

Brand names: Plegridy

Peginterferon beta-1a is a pegylated, long-acting interferon beta used as a disease-modifying therapy for relapsing-remitting multiple sclerosis.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 125 micrograms every 2 weeks (14 days), after titration: 63 micrograms at dose 1 (day 0), 94 micrograms at dose 2 (day 14), then the full dose of 125 micrograms from dose 3 (day 28) onwards

Route: Subcutaneous injection using a single-use pre-filled pen or pre-filled syringe, OR intramuscular injection using a single-use pre-filled syringe. Usual subcutaneous sites are the abdomen, arm and thigh; the usual intramuscular site is the thigh. Rotate injection sites every two weeks.

Frequency: Every 2 weeks (14 days)

eMC §4.2 (Plegridy 125 micrograms solution for injection in pre-filled pen). Treatment should be initiated under the supervision of a physician experienced in the treatment of multiple sclerosis. TITRATION: it is generally recommended that patients start subcutaneous or intramuscular treatment at 63 micrograms on day 0, 94 micrograms on day 14, and 125 micrograms (full dose) on day 28, continuing at 125 micrograms every 14 days. For the subcutaneous route an initiation pack contains the first 2 doses (63 micrograms, orange label; 94 micrograms, blue label; 125 micrograms, grey label). For the intramuscular route the dose pack contains the full 125 microgram dose in one pre-filled syringe, and single-use titration clips limit the delivered dose to 63 micrograms (dose 1, half dose, yellow clip) and 94 micrograms (dose 2, three-quarter dose, purple clip) on day 0 and day 14 respectively; each clip is used once then discarded with any remaining product, and no clip is needed from day 28. Dose titration at initiation may help ameliorate flu-like symptoms; prophylactic and concurrent anti-inflammatory, analgesic and/or antipyretic treatment may prevent or ameliorate them. ROUTE SWITCHING: switching between subcutaneous and intramuscular routes has not been studied, but based on demonstrated bioequivalence between the two routes, dose titration is not expected to be required when switching. MISSED DOSE: administer as soon as possible - if 7 days or more remain to the next planned dose, give the missed dose immediately and continue with the next scheduled dose as planned; if less than 7 days remain, begin a new 2-week dosing schedule from the missed dose given. A patient should never receive two doses within 7 days of each other. ELDERLY: safety and efficacy in patients over 65 have not been sufficiently studied due to the limited number of such patients in clinical trials. HEPATIC IMPAIRMENT: not studied. PAEDIATRIC: safety and efficacy in children and adolescents aged 10 to under 18 years have not been established in multiple sclerosis and NO posology recommendation can be made; safety and efficacy below 10 years have not been established and no data are available - verify any under-18 use against a children's formulary. HANDLING: allow to warm to room temperature (up to 25 degrees C) for about 30 minutes after removal from the refrigerator; do not use external heat sources. Do not use if the liquid is coloured, cloudy or contains floating particles; do not use the pre-filled pen unless the green stripes are visible in the injection status window. No US openFDA label was captured in this bundle.

Dose adjustments

Renal

eMC §4.2: no dosage adjustments are necessary in patients with renal impairment, based on study data in mild, moderate and severe renal impairment and end stage renal disease.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Contraindications

Hypersensitivity to natural or recombinant interferon beta or peginterferon, or to any of the excipients (eMC §4.3)
Patients with current severe depression and/or suicidal ideation (eMC §4.3)

Side effects

Influenza-like illness - very common (the most commonly reported reaction leading to discontinuation, under 1%); pyrexia and chills - very common
Injection site erythema, injection site pain and injection site pruritus - very common; injection site inflammation - common; injection site necrosis has been reported with subcutaneous interferon beta
Headache - very common; depression - common; seizure - uncommon
Myalgia and arthralgia - very common; asthenia - very common; hyperthermia - common
Nausea - common, vomiting - common, alopecia and pruritus - common; uncommon thrombocytopenia, angioedema and urticaria; rare thrombotic microangiopathy including thrombotic thrombocytopenic purpura/haemolytic uraemic syndrome, nephrotic syndrome and glomerulosclerosis; anaphylaxis and pulmonary arterial hypertension frequency not known

Clinical monograph

How it works

It is an immunomodulatory cytokine whose pegylation prolongs its half-life; interferon beta exerts complex effects on immune cell activity and cytokine balance that reduce relapse frequency, though the precise mechanism in MS is not fully understood.

Prescribing in practice

It can cause hepatic injury and bone-marrow suppression, so liver function and full blood count should be checked before and periodically during treatment.
Injection-site reactions and flu-like symptoms are common, particularly early on, and may be eased by gradual dose escalation and pre-emptive antipyretics.
Use with caution where there is a history of depression or seizures, and review for thrombotic microangiopathy or nephrotic syndrome if relevant symptoms arise.

Monitoring

Monitor full blood count, liver function tests, thyroid function and mood, and review for injection-site and systemic reactions during therapy.

Counselling the patient

Flu-like symptoms are common at first and usually settle; paracetamol or an anti-inflammatory taken around injection time can help.
Rotate injection sites and report any persistent low mood, jaundice, or unusual bruising.
Administer it on the same schedule as advised and store it correctly in the fridge.

Evidence & guidelines

The ADVANCE trial demonstrated reduced annualised relapse rates versus placebo, supporting its use as a disease-modifying treatment in relapsing MS.

Reference: NICE TA624; ABN MS-DMT prescribing guideline; SmPC; Confirm identity and dosing against the manufacturer SPC (eMC) and NICE. Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

Calculators

Pathways